Titanium has been widely used, for example, in aircraft, spacecraft, and chemical plants, because of its lightness, great mechanical strength and superior corrosion resistance. Since titanium parts for the above uses are mostly large, they can be produced by methods other than casting, for example, by forging. However, smaller parts, in particular those with complicated shapes such as prosthetic appliances, cannot be produced by methods other than casting.
When cast, an ingot of titanium or alloy of titanium must be speedily and efficiently melted. Since titanium has a high melting point of about 1,700.degree. C. and is very reactive at high temperatures, its melting and casting have presented heretofore unsolved problems with respect to the crucible and mold that can be used. It has been impossible to cast such particularly small parts with complicated shapes from titanium or alloys thereof because cavities are produced in the cast product, and the molten metal does not adequately fill the casting chamber of the mold. For example, if a crucible made of silica and alumina is used for melting titanium, the molten titanium readily reacts with silica (SiO.sub.2) or alumina (Al.sub.2 O.sub.3) to erode the crucible. In addition, titanium becomes oxidized when it comes into contact with the surface of the crucible, resulting in the formation of a fragile layer of titanium oxide (TiO.sub.2) on the surface of the titanium. When this titanium which contains titanium oxide impurities is cast, fragile portions are formed in the cast product.
Another disadvantage in the casting of titanium or alloys thereof is that certain properties of the mold, such as its corrosion resistance and heat resistance, are affected by the contact with molten titanium. For example, conventional molds made of high temperature materials such as phosphate, ethyl silicate and amorphous silicate are inevitably affected by contact with molten titanium and, as a result, a rough skin is produced on the surface of the cast product. This rough surface makes casting unsuitable for producing prosthetic appliances and other products to be used in an oral cavity.
In the casting of particularly small parts such as prosthetic appliances, the small space within the casting chamber of the mold makes it difficult to rapidly discharge air and gases from the casting chamber in order to reduce the build-up of air pressure therein and prevent molten titanium from flowing out. Furthermore, titanium has a high melting point of 1,720.degree. C., whereas the mold is held at relatively low temperatures (20.degree. to 600.degree. C.) in the casting process in order to prevent the formation of rough skins on the cast product. Thus, there is a remarkably large difference between the temperature of the molten titanium and the temperature of the mold (which difference is about 1,100.degree. to 1,700.degree. C.). Thus, the time between the start of casting of molten titanium to its solidification in the mold is remarkably short, often causing insufficient casting. The molten titanium is solidified before it is uniformly distributed through the inside of the mold, in particular in the depth of the mold, and the shape of the cast product does not correspond accurately to the shape of the mold.